The present invention relates to a semiconductor device for use in electrical products, such as an electric locomotive, home electric equipment, and so forth; and, in particular, the invention relates to improvement of a current sensing function provided in a semiconductor device.
In order to reduce the energy consumption in electric device, such as an inverter, it has been proposed to decrease the power loss in an insulated gate bipolar transistor (hereafter referred to as IGBT) of the type used as a switching element in an inverter. Recently, an IGBT in which gate electrodes are buried in a silicon substrate, which is called a trench gate type IGBT, has been actively developed.
In a trench gate type IGBT, it is possible to increase the components per chip to a higher level in comparison with a conventional planar gate type IGBT. Consequently, the voltage decrease which occurs during a current applying state in a chip, what is called the ON voltage, is smaller than that in a conventional planar gate type IGBT, and so the power loss of the device also can be reduced. However, a trench gate type IGBT has a problem in that, since the saturation current becomes larger because of the high number of components per chip, and an over-current flows in the case of a fault, such as a short circuit of a load, it breaks down more easily than a planar gate type IGBT.
A counter-measure for solving this problem is to provide an over-current protection circuit for detecting an over-current flow in the case of a fault to protect the IGBT. However, a highly accurate current sensing function is necessary for such an over-current protection circuit. Especially, in a trench gate type IGBT, since the increase in the rate of the current in a turn-on state is large, and a large current flows quickly to damage the IGBT, an accurate and high-speed over-current protection function is required.
FIG. 3 shows an equivalent circuit of an IGBT having an over-current sensing function. The IGBT having an over-current sensing function is composed of a main IGBT 300, a sense IGBT 301, a collector electrode 121 commonly used for the main IGBT 300 and the sense IGBT 301, a gate electrode 106, an emitter electrode 122 of the main IGBT 300, and an emitter electrode 120, which is provided separately from the emitter electrode 122 and is referred to as a sense electrode.
Generally, a sense IGBT is designed so that the number of cells of the sense IGBT is approximately 1/1000 of that of cells of the main IGBT. Consequently, the sense IGBT can extract a current flow of about 1/1000 of a current flow in the main IGBT. By monitoring such a small current flow, the sense IGBT can monitor the state of a large current flowing in the main IGBT. However, actually, the ratio of the small current flow in the sense IGBT (referred to as a sense current flow) to the large current flow in the main IGBT (referred to as a main current flow) is not always equal to the ratio of the number of cells of the sense IGBT to that of cells of the main IGBT, and this current flow ratio changes easily, corresponding to the collector voltage, the temperature in the IGBTs, etc. For a planar gate type IGBT, the number of structures necessary to attain a stable current flow ratio has been devised. For example, such a structure is disclosed in a paper: "Current Sensing IGBT for Future Intelligent Power Module" by M. Kudoh et al., Proceedings ISPSD, pp. 303-306, 1996.
In FIG. 4, an example of a layout of cells of a sense IGBT in a planar gate type IGBT is shown. By arranging the cells 401 of the sense IGBT as shown in FIG. 4, an improvement in the accuracy of the current sensing operation is attained, or by providing a separation region between the cells of the sense IGBT and those of the main IGBT, the accuracy of current sensing is improved.
However, the inventors have found that even when a layout of cells of a sense IGBT, which has been adopted for a conventional planar gate type IGBT, is applied to a trench gate type IGBT, it does not improve the current sensing accuracy of the sense IGBT of the trench gate type.
One of the reasons for this is that the current sensing accuracy of a sense IGBT is degraded by the variation in shapes of trench gates in a sense IGBT, which variation is produced during element processing and causes differences from the shapes of the trench gates in a main IGBT. Further, the current sensing sensitivity of a sense IGBT is degraded because of the interaction between cells of a sense IGBT and those of a main IGBT.